1. Field of the Invention
This invention relates to heart pacers, and more particularly to heart pacers of the type of which the operating parameters can be remotely varied and controlled.
2. DESCRIPTION OF THE PRIOR ART
Heart pacers are often provided with various means to control selected operating parameters so that, for example, the pacer can be adjusted to respond to and provide signals to a patient in accordance with his individual needs. After the pacer has been implanted, however, effecting changes in any particular operating parameter of the pacer may become a severe problem. Such changes are often necessitated, for instance, by an improvement or deterioration in the patient's condition which may require certain adjustments, for instance, that rate at which stimulation pulses are provided to the patient, the pulse width and amplitude of the pulses provided, the refractory period exhibited by the pacer, the sensitivity to naturally produced heart signals, and the operating mode of the pacer, such as demand mode, fixed rate mode, and the like.
In the past, there have been various proposals to effect changes in selected heart pacer parameters of an implanted heart pacer without major surgical intervention. For example, it has been proposed to provide a magnetic field responsive switch which can be activated by a magnet external to the patient, typically to change the pacer from a demand mode to a fixed rate mode. Additionally, pacers have been proposed which provide receptacles for receiving engaging surgical needles which can be inserted into the patient for adjustment of, for example, the value of a resistor or the like.
Implantable cardiac pacers have been advanced in the prior art which have certain operating parameters which can be adjusted without physically invading the user's body. Some prior art pacers typically include first and second digital pulse counters. The first counter receives a first predetermined set of input pulses, after which it produces an enabling signal to direct subsequent input pulses to the second counter. The second counter includes a plurality of outputs to activate various switches, each in parallel, for example, with a respective resistor in a plurality of series connected resistors to thereby control the input current to a transistor of a multivibrator to modify its time constant. Another output of the second counter controls a switch which bypasses a portion of the driving current to the output amplifier of the pacer to thereby control its output current.
Such prior art circuit, as above described, typically responds to an access code, which may be merely a sequential series of, for instance, seven or more consecutive pulses. One requirement of such circuit, however, is that seven access code pulses appear in a series to enable the first counter to count them. Thus, for example, if the wearer of a cardiac pacer embodying such circuit were to become exposed to a field to which the circuit responds, the circuit may erroneously detect the field pulses as the access code, and more importantly, thereafter reset the various parameters controlled by the circuit.
Additionally, prior art parameter adjustment circuits typically provide means for modifying the pulse rate only (and incidentally the width) and output current produced by the pacer, with no consideration given to many of the other important pacer parameters above mentioned.
One other disadvantage of prior art parameter adjustment circuts is that once the access pulses have been counted and the second counter is enabled, the data pulses applied to it produce immediate changes in the various output lines. These changes may undesirably produce immediate changes in the selection of the various timing resistors during the time while the data is still being received.